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The Physics of Clay Bodies

Section: Clay Bodies, Subsection: Testing

Description

Learn to test your clay bodies and recording the results in an organized way and understanding the purpose of each test and how to relate its results to changes that need to be made in process and recipe.

Article

If you have been working with glazes and glaze chemistry for some time, you may have developed a mindset that is too narrow when it comes to dealing with clay body formulation. Clay body analysis is much more of an adventure in mineralogy and physics than it is in oxide chemistry. Two clays of completely different physical properties can have very similar chemistry; two clays of radically different chemistry can have very similar physical and fired properties. Thus on the surface it would seem that chemistry is of little use in formulating and evaluating clay bodies. Actually, this is not quite the case, but it is not far from it.

When glazes melt everything usually goes into solution, but the vitrification process of a clay is quite different. The differences in mineralogy, particle size, firing history, body preparation, and ware forming methods all influence the final fired product. Thus the ability to measure physical clay properties is very important. In this section of the book, I am going to introduce you to some very simple clay tests that you can learn and do. They do not require advanced test equipment and they tell you an incredible amount about a material.

There is no time to waste in learning how to effectively test your clay bodies and materials. In this chapter I'll outline how you can go about getting a quality control program going on a low budget. Don't let anyone tell you that modern test equipment has supplanted this type of test. You can go into a lab full of million dollar test devices and ask the technicians to describe to you exactly what clay is and I'll bet few could do it in an understandable way that relates to the key reasons why we use clay in ceramics, namely plasticity and vitrification. They could likely show you thousands of numbers from DTA, CoE, XDF, etc. machines, but these are comparative measurements used for quality control, and technicians often lose sight of the reason some properties are even measured. While machines don't measure plasticity well and lab techs don't describe it well (the average potter could talk about the subject at length), it is directly related to shrinkage. The other reason for using clay is that it forms a rock when heated. Porosity and fired shrinkage measurements tell you how complete that process was.

Let us start quickly. Consider the three simple-looking specimens that make it all possible.

These three tests can completely change your view of clay bodies

As you will see, by making the above test specimens for a clay, you will be able to record its absorption and shrinkage over a range of temperatures, its water content, density, dry shrinkage, loss on ignition, soluble salts content, drying performance, glaze-over behavior, and dry strength. While these tests require very little investment in equipment (assuming you already have a gram scale and calipers), there is one testing device you really should buy: a good set of sieves. I will consider these in a separate section.

By doing these tests in a very standarized way, your data will be universal to all other tests that both you and others do (I'll explain what I mean by 'standardized' in a minute). It means you can compare clay properties using real numbers.

As most people have learned, glazes don't travel well. Still, we can compensate for this somewhat with calculations that attempt to preserve a glazes oxide formula into a new setting. But with clay bodies the added dimension of physical properties demands center stage. Unless you can test for them, you cannot even adjust a body let alone 'take it on tour'. For example, while you can usually exchange one kaolin for another in a glaze, such adjustments are likely to have considerable effects on a clay body's drying performance, green strength, fired color, and casting behavior, to mention only a few. Even changing the particle size of a constituent body material can have significant impact.

>This subject reveals an interesting comparison between potters and industrial technicians. On one hand, the potter judges a body by how it feels in his hands, how it bends, stretches, pulls, how it behaves on the wheel, how it trims, or how successfully it dries with his ware and techniques. He evaluates it on how it reacts visually with his glazes and fires in his kiln; he dynamically adjusts procedures to compensate for changes he is able to perceive. On the other hand, a ceramic engineer may have never handmade a piece of pottery in his life. As a result, he may not fully appreciate what plasticity is, viewing it merely in terms of how the clay reacts in machines. To him, dried and fired properties exist as numbers produced by test equipment.

Potters often have excellent all-around knowledge; some have remarkable intuitive abilities at evaluating clay bodies; they like to look down at engineers whose cold numbers and charts keep them at a distance from the material. Some potter's textbooks are incredibly insightful and helpful. Yet there is no denying the value of good physical properties testing and hard test results. The ideal is probably a situation somewhere in between these two extremes. Many body properties are immediately evident in the hands of an experienced potter and not quickly shown by instruments. Likewise, differences shown by instruments can explain strange results in the potter's kiln.

There are still many companies in the ceramic industry that do not have a standard testing and quality control program in place. The question is, where does one start to test his clay bodies; how do you set up a simple but relevant program? One answer is FORESIGHT Ceramic Database software. It provides a way to define your own test procedures, variables, and equations in keeping with equipment you have. It acts as a platform from which to accumulate unlimited test results and allows you to search, query, and report these results as needed. Being a mature software solution it has a better chance of success than any effort to date.

In a few minutes, I will show you some reports generated by FORESIGHT for one of its predefined tests. But first, let us review the options you have with regard to setting up a test program.

Testing Categories:

Universal Standards

An example is the 50-volume Annual Book of worldwide ASTM Standards (American Society for Testing and Materials, 1916 Race St, Philadephia, PA 19103). One of the volumes deals with refractories, glaze, and ceramic materials. The books are well organized and describe all test procedures in great detail. Just reference a test by number and you convey all details about how you achieve your results.

A customer will sometimes require that a manufacturer document quality and compliance of each product shipment. In this case, the client may reference a standard test or define his own test procedure for the manufacturer to carry out. With the advent of quality control standards like ISO 9000, customers are going to the next step and requiring documentation not only on how tests are done, but tolerances, noncompliance procedures, procedure change mechanisms, test equipment calibration schedules, and proof of certification.

Internal

Many tests are internal to a company, intended to solve problems, maintain properties critical to production efficiency and cost, control reject rates, etc. In this situation, the manufacturer is quite free to formulate any method that seems best for the circumstances.

Tests have typically required expensive equipment. In the real world, technicians generally have to make do with what is available, so standard methods are usually adjusted. This is not necessarily bad. Simple tests are sometimes most revealing (excellent examples are the FORESIGHT DFAC and SOLU tests). It is important to think a test through thoroughly, document it, and analyze the information it provides. If you can prove the value of the information, customers will respond positively and production yields and quality will improve.

Implementing a Test

Define the testDecide what physical properties need to be measured, and if possible, take an existing test procedure (like the FORESIGHT SHAB test shown below) and redefine it for your needs. If possible, formulate the test to measure as many physical properties as possible. For example, one test bar can be used to measure dry shrinkage, fired shrinkage, and absorption.

Document the testUsing the pattern provided in the SHAB test, clearly set out the reason for the test, the physical properties it will measure, the procedure, and how the results will be used.

Set up the softwareSet up your variables in FORESIGHT, print data entry forms, accumulate test results, print reports in the required format for individual tests, and track testing and problem histories.

Put the test into practice as documentedCarry out the test as defined on a trial basis, make the needed changes, and update the documentation until the bugs have been worked out.

Accumulate tolerance samplesWhere a test involves making a subjective observation, accumulate samples that demonstrate the tolerances. For example, if you must record the relative amount of soluble salt discoloration on the fired surface, gather samples to show the worst tolerable amount.

Analyse the results and take corrective actionWhen test data is accumulated on computer, there is a real danger that the staff will just go through the motions of collecting the information and no one will ever do anything with it. Fine tune the analysis aspects of the test procedure to make sure that at some point, test results are being compared with standards, decisions are made, and actions are taken according to these comparisons. Make sure the procedure definition includes provision for trend reports and historical analysis to help improve plant performance.

FORESIGHT predefines many tests and the ones of interest to us here are the SHAB (Shrinkage, Absorption), DFAC (Drying Factor), SOLU (Solubles), and LDW (LOI, Density, Water Content). The procedures for these four describe how to make and process the three simple specimens I showed you at the beginning of this chapter (shrinkage bars, H2O bars, drying disk). They also provide a framework within which to gather data.

This is the first part of the Test Procedure report for the SHAB test. It is formatted like a standard ISO 9000 style procedure.

PROCEDURE NO 02-012-002
REVISION NO: 2
DATE: 03/16/97
PAGE 1 OF 8

Title: SHAB - SHRINKAGE/ABSORPTION/H2O

1. Purpose of Test

1.1 This test is designed to measure dry shrinkage, absorption
and fired shrinkage properties. Results from this test are
repeatable if instructions are followed closely.

1.1.1 DRY SHRINKAGE
As a clay dries the removal of interparticle water causes the
mass to tighten up and pack together resulting in shrinkage.
Clays of fine particle size and those of high plasticity have
high shrinkage. Unfortunately the benefits of plasticity are
offset by drying problems. Variation in drying shrinkage is an
indicator of changes in a clays plasticity. However comparing
the dry shrinkage of different types of clay is not necessarily
in indicator of their comparative plasticity since some fine
clays are not plastic. Note that higher water content also means
greater dry shrinkage.

For typical modeling stiffnesses dry shrinkage for non-plastic
clays is around while plastic clays which require care in drying
are usually above 7.0%. High shrinkage can be reduced by the
addition of an aggregate however this can produce a matrix where
micro-cracks radiate outward from each of these larger particles
creating a weaker dried and fired product. A low drying
shrinkage is important to successfully dry larger items or ware
of uneven cross section.

Dry shrinkage is simply the per cent change in length between
wet and dry. The SHAB test provides the data for this property
as follows:

Wet length - dry length / wet length * 100

or where a 10 cm marks are stamped on the bar it is simply:

100 - mm dry length

1.1.2 FIRED SHRINKAGE
As a clay fires, it shrinks and particles continue to pack
together. At some point, they begin to break down and react with
each other, fluxes begin to melt and flow, and mineral grains
seed the development of more stable forms. The amount of
shrinkage during firing is thus an indication of the degree to
which the complex "maturing" process has proceeded.

This report shows the variables defined for the SHAB test. Data is to be collected for each variable. It also displays calculated fields and the equations used to derive them from the variable data.

Purpose: This test is designed to derive shrinkage and absorption data
by drying and firing clay bars according to a detailed procedure. This
test is recommended over the SAWL test since its fewer variables mean
that you can route results reports to the screen and they will fit
within 80 columns. This test does not account for LOI as does SAWL,
however it is assumed that you will also use the LDW test.

Much of the theory behind why this test is beneficial is dealt with in
the Digitalfire book "The Magic of Fire II". Like other tests defined here, it
is assumed that you have the FORESIGHT software to log results.

This test is meshed with the SOLU, DFAC and LDW tests in that it is very
convenient to perform all four at the same time.

1) Dry Length...................DRY-LEN " 1"
The distance between the outer edges of
two marks on the dried clay bar as
measured with a set of calipers. These
marks were pressed into the wet bar at
exactly 10 cm apart on the outer edges.

2) Fired Length.................FIR-LEN " 2"
The length between two marks on the
fired clay bar as measured with a set of
calipers.

3) Fired Weight.................FIRE-WT " 3"
The weight in grams of the clay bar
after firing.

4) Boiled weight................BOIL-WT " 4"
The weight in grams of the clay bar
after boiling for 5 hours and soaking
for 19 and being blotted on a towel.

5) Fired cone...................CONE " 5"
The Orton cone number to which the bar
was fired. Take the highest cone to show
deformation and interpret it as follows:

The end-product of all your clay body testing work is to generate 'real numbers' that mean something;reliable numbers that can be compared with others to reach conclusions. While the above report may appear a little foreign, it all comes together when you see it in terms of the structured set of variables which are defined for each test. This is a basic report showing gathered data and the results of equations applied to that data. But it is a beginning of a flexible testing system on which much more graphical reports can be built.

So my advice is simple. Set up a little lab for yourself and take control of the physical properties of your clay bodies and materials.

Pictures

A batch of fired clay test bars in the Plainsman Clays lab

A batch of fired test bars that have just been boiled and weighed, from these we get dry shrinkage, fired shrinkage and porosity. Each pile is a different mix, fired to various temperatures. Test runs are on the left, production runs on the right. Each bar is stamped with an ID and specimen number (the different specimens are the different temperatures) and the measurements have all be entered into our group account at insight-live.com. Now I have to take each pile and assess the results to make decisions on what to do next (documenting these in insight-live).